Freshwater Fish

Encyclopedia of Food and Culture
COPYRIGHT 2003 The Gale Group Inc.

Freshwater Fish

Fish have been a major source of human food and of oil, fertilizer, and feed for domestic animals since the dawn of history. Efforts to propagate fish as a source of high-grade protein for human consumption have been more recent but still date to ancient China and the Roman Empire. In the twenty-first century, fish provide about 25 percent of the animal protein consumed by people in developing countries and as much as 75 percent in countries such as Bangladesh and the Philippines.

One reason for the long-standing popularity of fish as food is sheer numbers. More than 70 percent of the Earth's surface is covered with water, and well over twenty thousand different species of fish live in marine, fresh, and brackish waters, making them the most diverse of all the animals. Fish can live at temperatures ranging from below freezing in Antarctic waters to over 100oF (40oC) in hot springs. They range in size from the .5-inch (1.3-centimeter) dwarf goby of the Philippine Islands to the 45-foot (14-meter), 25-ton whale shark of the tropical oceans. The nutritional profile of fish is also outstanding. Rich in the essential omega-3 unsaturated fatty acids so lacking in other foods, fish are also high in protein yet low in calories, sodium, sugars, saturated fats, and cholesterol.

Biology

Scientifically speaking, fish are aquatic vertebrates with gills instead of lungs and fins instead of external limbs. In contrast to the higher animals, fish are also cold-blooded, that is, their body temperatures remain the same as that of the water. A few species, such as tuna, are able to maintain their body temperatures a degree or two higher than the water.

The chain of life leading to fish production begins with the microscopic diatoms and algae in lakes, rivers, and the ocean. These aquatic plants, collectively termed phytoplankton, use the energy in sunlight to convert carbon dioxide dissolved in the water into the organic matter that eventually becomes food for fish. Fish were the earliest animals with backbones to appear in the fossil record, evolving from more primitive forms over 500 million years ago. In turn, the terrestrial animals evolved from the fishes.

Biologists class the more than twenty thousand known species of fish into three main groups, the Agnatha (primitive jawless fishes, such as the blood-sucking lamprey), the Chondrichthyes (sharks, skates, and rays that have skeletons of cartilage instead of bone), and the Osteichthyes (fishes with a bony skeleton, such as salmon and trout). All of the fish important as food are members of this latter group.

A typical bony fish is torpedo-shaped with a head containing a brain and eyes, a trunk with a muscular wall, and a postanal tail. Fish generally propel themselves through the water by undulating movements of the muscular trunk, using their fins to control direction. All have skins covered with a layer of mucus that decreases friction with the water, and nearly all are covered with an external layer of scales (catfish are one exception). Fishes also have a system of sensory organs along their sides, called the lateral line, that can detect pressure changes in the water caused by sounds. Fish obtain oxygen and eliminate carbon dioxide (breathe) by sucking water into the mouth and pumping it out over the gills. Oxygen dissolved in the water thus diffuses into the bloodstream, and carbon dioxide diffuses out. A few species (such as the African lungfish) also have air-breathing lungs as an additional means of respiration.

Most fish live in either saltwater or freshwater, but some important food fish are physiologically capable of migrating from one to the other. For example, Pacific and Atlantic salmon are hatched and reared in freshwater but then migrate to the ocean to grow and mature, returning to their natal streams and lakes to spawn. The eel has the opposite life history pattern. Thus eel and salmon may be thought of as either freshwater or saltwater fishes depending on age and season.

Over the years, a number of other aquatic animals have been given common names that include the term "fish," such as shellfish, but these do not resemble and are not related to true fish. Furthermore, some animals that have adopted an aquatic way of life, such as whales, seals, and sea snakes, superficially resemble fish and may even be called fish. But they are air breathers, and their anatomical structure is that of land animals.

Preparation and Food Safety

Fish are a highly perishable food product, and historically they had to be marketed live or preserved (cured) by smoking, salting, pickling, or a combination of these methods. Fish to be cured by any method are first cleaned, scaled, and eviscerated. They are salted by packing them between layers of salt or by immersion in brine. Smoking preserves fish both by permeation of smoke ingredients and by partial drying due to heat penetration. Fish can also be dried per se by carefully controlling temperature, humidity, and air velocity.
However, dried fish are relatively unappetizing, and rehydration is slow. With the exception of smoked fish, the ready availability of ice and modern freezing and canning facilities has largely supplanted curing as a method of fish preservation. Fish are routinely shipped around the world either fresh or frozen. Fresh fish are shipped on ice and have an acceptable shelf life of about ten days. Frozen fish packaged in oxygen-impermeable plastic wrap, such as Saran, may be stored frozen at –20oF (–29oC) for up to six months with no appreciable loss in quality.

Fresh fish are almost always marketed as either whole fish on ice (viscera removed), dressed fish (head, fins, and viscera removed), fillets (sides cut lengthwise away from the backbone), or steaks (cut longitudinally into sections). Due to consumer demand, boneless cuts are increasingly available in the United States and Europe.

Fish is a naturally tender protein food, free of tough fibers that need to be softened by prolonged cooking. Thus fish products are best cooked using high-temperature, short-time methods. They may be deep-fat fried (325–350oF; 163–177oC), pan fried (sautéed) in a small amount of butter, broiled, poached (simmered, never boiled), or baked (400–450oF; 204–232oC). Pan frying or sautéing is one of the most widely used methods of cooking thin fillets in general. Microwaving is especially well suited to the high-temperature, short-time method of cooking fish. The advent of individually quick-frozen fish fillets has enabled timesaving cooking techniques, such as brushing marinades directly on the frozen product and grilling or oven roasting without the necessity of defrosting. Fish is generally ready to eat when cooked to a temperature of 140oF (60oC) and the flesh has turned opaque and flakes easily. Fish is eaten raw by some ethnic groups (such as Asians). Other ethnic specialty preparations, such as blackened fish (Cajun) or gefilte fish (Yiddish), are also popular.

To ensure food safety, fresh fish should be clean smelling, and the flesh should be firm and resilient when pressed. Fish should be kept wrapped and refrigerated at 40oF (4oC) or less and eaten within two days. Frozen fish should be rock hard, free of ice crystals, and have no white spots, visible drying, or browning around the edges. In the home, fish should be stored frozen at 0oF (–18oC) or below and for no more than three months. It should be thawed in a refrigerator, never at room temperature.

Freshwater Fish Commonly Used as Food

Historically, the human race has used literally thousands of different species of fish in its continuing search for sustainable sources of food. In the twenty-first century, the most popular in North America and Europe include carp, catfish, crappie, eel, lake herring, mullet, muskellunge, yellow perch, yellow pike, pickerel, salmon, suckers, sunfish, tilapia, trout, lake trout, and whitefish. In Indonesia, the Philippines, and Taiwan, milkfish have been used for food for centuries. In Asia, carp, ayu, and eel are important freshwater food fish. Some of the most interesting of these freshwater fish are discussed in more detail below.

Ayu. The ayu (Plecoglossus altivelis), also known as sweet fish in Japan and aroma fish in China, is an extremely popular and economically important freshwater food fish in many Asian countries. Historically, it was caught by Japanese fishermen using trained cormorants with rings around their necks to prevent them from swallowing. In the twenty-first century, it is wild-caught in rivers by sport and commercial fishermen or raised commercially for both restaurant consumption and home use. Ayu are usually sold live, on ice in the round, or frozen. The food quality of wild-caught ayu is especially desirable, characterized by a sweet, delicate taste and an odor reminiscent of cucumber or watermelon.

Carp. Carp (Cyprinus carpio) are the largest members of the minnow family and can easily reach a weight of ten kilograms or more. Although greatly underutilized in North America, the common carp has always been a widely popular freshwater food fish in the rest of the world. History records that carp were grown in ponds for food in ancient China in the fifth century b.c.e. In Europe, carp were grown in monastery ponds as early as the sixth century C.E. so the monks would have something to eat during the many meatless fasting days prescribed by the church. By the late Middle Ages, carp had become a well-established food item for the general populace. In the twenty-first century, carp are wild-caught or grown for food in Russia, Ukraine, Hungary, Poland, India, China, Japan, Latin America, Egypt, Iran, Indonesia, and Israel, to name only the major consumer nations. The world's leading producer is China, where carp are often grown in rice paddies in rotation or even simultaneously.

The most common market forms of carp are fresh whole fish, dressed fish, or fillets. Gefilte fish, fish balls blended with egg and matzo meal and simmered in a vegetable broth, is an ethnic specialty item (Yiddish) traditionally made from carp.

Catfish. The channel catfish (Ictalurus punctatus), native to warm water lakes and rivers in North America, is a traditional food fish in the southern United States. Consumer demand has moved from regional to national and even international. In the United States, the per capita consumption of catfish is exceeded only by that of tuna, shrimp, pollack, and salmon. To satisfy American consumer demand, several hundred thousand metric tons of channel catfish are produced by aquaculture each year in the southern United States.

Imported catfish from Vietnam has been marketed aggressively to restaurant chains and food service companies with considerable success. Advertised as delta-raised catfish, it is actually a catfish relative raised in the delta of the Mekong River. Another catfish species, the walking catfish, is a popular food fish in tropical regions and even in some European countries, especially the Netherlands.

Catfish is firm textured and has a mild, slightly nutty taste that complements a variety of flavors. It is a lean fish, and modern processing methods have eliminated bones. That, together with its lack of a fishy odor, gives it wide consumer appeal. Catfish were traditionally wild-caught and marketed as iced whole dressed fish. Modern farm-raised catfish are processed within minutes and shipped either on ice or as individually quick-frozen fillets, making it one of the freshest fish available. In addition to fresh or frozen fillets, steaks and nuggets (pieces) breaded or marinated with flavors and spices, such as Cajun spices or mesquite, are also common in seafood markets and restaurants and have even been introduced into school lunch programs. As Mark Twain once said, "The Catfish is a plenty good enough fish for anybody."

Eel. Although appreciated before the Civil War in North America, freshwater eels (primarily Anguillaanguilla and A. japonica ) are a widely popular food item in Asian countries, particularly Japan, Korea, China, and Taiwan. Eels are also an important delicacy in Europe, particularly Italy, where they must be produced commercially by aquaculture to satisfy consumer demand. Overall, however, China produces more than 70 percent of the eels sold in the world, and many rice paddies have been converted to eel production. Japan is the world's largest eel consumer, where kabayaki, eel fillets grilled with a sweet basting sauce, is practically a national dish.

As mentioned, eel consumption in North America is minor. However, freshwater eel unagi is common in Japanese restaurants in the United States, where it may be served grilled with teriyaki sauce or used in sushi or unadon (eel over rice). In addition, each year many tons of market-sized eels are wild-caught by U.S. fishermen and exported to Europe, where it is eaten roasted or even jellied and baked into pies.

Eels have an interesting life history in that they live in freshwater rivers and lakes, where they grow to adult size, then they migrate into the ocean, where they swim long distances to the Sargasso Sea to spawn and die. The newly hatched young eels may then ride the ocean currents for several years until they reach coastal waters and swim back to freshwater rivers. There they grow to adult size and can be harvested for food.

Eels under a kilogram in size are the most tender. The rich, sweet, firm flesh of eel must be refrigerated and eaten immediately, so the best restaurants keep live eels in aquarium tanks. The skin and outer layer of fat are removed by the chef, and the fillets are either grilled or roasted. Eel is also available frozen, smoked, or jellied in cans.

Milkfish. Milkfish (Chanos chanos) have been an important food fish for people in Southeast Asia for many
centuries. Although they are an oceanic fish, milkfish spawn in shallow coastal areas, where fry and fingerlings are collected in nets and carried to freshwater or brackish water ponds for rearing to market size. Milkfish have been raised in this fashion for at least seven hundred years in the Philippines and Indonesia. Taiwan is also a major producer. Milkfish (bangus) is sold in Asian markets and restaurants either fresh, smoked, marinated, as fish balls, as fish sausages, or as fish nuggets. It is also exported frozen to North America, where sinigang na bangus (milk-fish in sour broth) is a popular dish among ethnic Indonesians and Filipinos.

Tilapia. Although relatively new to North American fish markets, tilapia are actually a group of fish (cichlid) that traces its origins to North Africa and the Middle East. These mild, white, sweetly flavored fish have been wild-caught or pond-raised around the world for centuries. Called St. Peter's fish in many parts of the world, legend has it that the fish Jesus multiplied to feed the multitudes in the story of the seven loaves and fishes was tilapia (Matt. 17:24–27).

Because of their versatility, tilapia have been nicknamed "the aquatic chicken" and can be baked, broiled, fried, blackened, grilled, poached, or sautéed. Sautéing is one of the most popular methods of preparing thin fillets in general, and in most recipes tilapia can easily substitute for catfish or even sole and flounder.

Tilapia are grown in floating cages, ponds, or rice fields in temperate and tropical regions around the world. Only Chinese carp and salmon or trout exceed tilapia in total worldwide fish production. Although they are less popular in the United States, tilapia consumption has grown to rival trout among the commercially raised fish species. Since relatively modest numbers of tilapia are produced by U.S. aquaculture, large quantities of frozen fillets are imported from Indonesia, Taiwan, and Mexico to satisfy consumer demand. Many large U.S. cities report a significant demand for live tilapia delivered to ethnic Asian markets.

Trout. Many trout species have historically been used for food, but rainbow trout (Oncorhynchus mykiss) have been by far the most popular. Originally native to cold water environments in the north temperate zone, this prized food fish has been transplanted around the world and is well established in North and South America, Japan, China, Europe, Australia, New Zealand, and parts of Africa. Top trout-producing countries include Chile, Denmark, France, Italy, and the United States.

Most rainbow trout is marketed as head-on dressed fish, as fresh or frozen boneless fillets, or as smoked fish. Farmed trout are typically rich in the omega-3 fatty acids so essential to normal brain and eye function, while they are less expensive than most other fish products.

Walleye pike. Walleye (Stizostedion vitreum), a member of the perch family with an excellent reputation for its food quality, is a widely sought cool-water fish mostly caught by anglers for home use but also available in fish markets and restaurants in much of the northern United States and Canada. In the United States, a limited commercial harvest comes from the Great Lakes. However, most of the commercial harvest is from Canadian fishing on Lake Erie and the inland waters of Ontario and Saskatchewan. The walleye is Canada's most economically valuable freshwater fish. Only a few commercial growers produce food-size walleye, but because of its reputation for excellent food quality (aroma, flavor, and texture), its name recognition, and its high retail price, walleye has considerable aquaculture potential. Traditionally, walleye are sold as scaled, skin-on fillets. A two-pound fish yields about two eight-ounce dinner-size fillets.

Whitefish. Lake whitefish (Coregonus clupeaformis) native to the deep cold lakes of North America are popular food fish in the United States and Canada. They are widely sold in restaurants, and some believe their flaky, non-oily white meat is the best tasting of all the freshwater fish. Early settlers claimed they could eat nothing but whitefish for days at a time and never tire of it. A large commercial fishery for whitefish exists in Lake
Superior and the other Great Lakes of the midwestern United States. In Canada, close to 600,000 kilograms of whitefish a year is caught and sold by tribal fishermen of the Great Slave Lake alone. Most whitefish is marketed frozen and sold in restaurants or supermarkets, but limited amounts are also available smoked or fresh. White-fish eggs, termed freshwater or golden caviar, are sometimes sold as a less-expensive substitute for sturgeon caviar.

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Biome

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Biome

A biome is an ecosystem containing plant and animal species that are characteristic to a specific geographic region. (An ecosystem is the community of plants and animals in an area considered together with their environment.) The nature of a biome is determined primarily by climate, including a region's annual average temperature and amount of rainfall. Biomes are often named for the vegetation found within them. They can be classified as terrestrial (land), aquatic (water), or anthropogenic (dominated by humans). Some familiar examples of biomes include tundra, desert, chaparral, and open ocean. The accompanying drawing shows the variety of biomes that can be found along just two lines of longitude on Earth's surface.

A number of attempts have been made to classify the world's biomes. One of the best known was proposed by the Russian-born German climatologist Wladimir Koeppen (1846–1940). Some of the biomes described by Koeppen are described below.

Terrestrial biomes

Tundra. A tundra is a treeless region in a cold climate with a short growing season. Most tundras receive little precipitation. Still, their soil may be moist or wet because little evaporation occurs. Loss of water by seepage is also prevented because the soil is frozen. Very little vegetation grows and very few animals live in the coldest, most northern, high-arctic tundras. These tundras are dominated by long-lived but short-statured plants, typically less than 5 to 10 centimeters (2 to 4 inches) tall. Low-arctic tundras are dominated by shrubs as tall as 1 meter (3 feet), while on wet sites relatively productive meadows of sedge, cottongrass, and grass grow. In North America, arctic tundras can support small numbers of plant-eating mammals, such as caribou and musk oxen, and even smaller numbers of their predators, such as wolves.

Words to Know

Anthropogenic: Resulting from the influence of human action on nature.

Aquatic: Related to water.

Benthic: Referring to the deepest parts of the oceans.

Boreal: Located in a northern region.

Conifer: Plants whose seeds are stored in cones and that retain their leaves all year around.

Deciduous: Plants that lose their leaves at some season of the year, and then grow them back at another season.

Ecosystem: An ecological community, including plants, animals, and microorganisms, considered together with their environment.

Eutrophic: A productive aquatic region with a large nutrient supply.

Herbaceous: A type of plant that has little or no woody tissue and usually lives for only one growing season.

Tropical: Characteristic of a region or climate that is frost free with temperatures high enough to support—with adequate precipitation—plant growth year round.

Upwelling: The process by which lower, nutrient-rich waters rise upward to the ocean's surface.

Wetlands: Areas that are wet or covered with water for at least part of the year.

Boreal coniferous forest. The boreal coniferous forest, or taiga, is an extensive northern biome occurring in moist climates with cold winters. The boreal forest is dominated by coniferous (cone-bearing) trees, especially species of fir, larch, pine, and spruce. Some broad-leaved trees are also present in the boreal forest, especially species of aspen, birch, poplar, and willow. Most boreal forests are subject to periodic catastrophic disturbances, such as wildfires and attacks by insects.

Temperate deciduous forest. Temperate deciduous forests are dominated by a large variety of broad-leaved trees in relatively moist, temperate (mild or moderate) climates. Because these forests occur in places where the winters can be cold, the foliage of most species is seasonally deciduous, meaning that trees shed their leaves each autumn and then regrow them in the springtime. Common trees of the temperate deciduous forest biome in North America are ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, tulip-tree, and walnut.

Temperate rain forest. Temperate rain forests are characterized by mild winters and an abundance of rain. These systems are too moist to support wildfires. As a result, they often develop into old-growth forests, dominated by coniferous trees of mixed age and various species. Individual trees can be very large and, in extreme cases, can be more than 1,000 years old. Common trees of this biome are species of Douglas-fir, hemlock, cedar, redwood, spruce, and yellow cypress. In North America, temperate rain forests are most commonly found on the humid west coast.

Temperate grassland. Temperate grasslands occur under climatic conditions that are between those that produce forests and those that produce deserts. In temperate zones, grasslands typically occur in regions where rainfall is 25 to 60 centimeters (10 to 24 inches) per year. Grasslands in North America are called prairies and in Eurasia they are often called steppes. This biome occupies vast regions of the interior of these continents.

The prairie is often divided into three types according to height of the dominant vegetation: tall grass, mixed grass, and short grass. The once-extensive tall grass prairie is dominated by various species of grasses and broad-leaved, herbaceous plants such as sunflowers and blazing stars, some as tall as 3 to 4 meters (10 to 13 feet). Fire played a key role in preventing much of the tall grass prairie from developing into open forest. The tall grass prairie is now an endangered natural ecosystem because it has been almost entirely converted for agricultural use.

The mixed grass prairie occurs where rainfall is less plentiful, and it supports shorter species of grasses and other herbaceous plants. The short grass prairie develops when there is even less precipitation, and it is subject to unpredictable years of severe drought.

Tropical grassland and savanna. Tropical grasslands are present in regions with as much as 120 centimeters (47 inches) of rainfall per year, but under highly seasonal conditions with a pronounced dry season. Savannas are dominated by grasses and other herbaceous plants. However, they also have scattered shrubs and tree-sized woody plants that form a very open canopy (a layer of spreading branches).

Tropical grasslands and savannas can support a great seasonal abundance of large, migratory animals as well as substantial populations of resident animals. This is especially true of Africa, where on the savanna range—among other animals—gazelles and other antelopes, rhinos, elephants, hippopotamuses, and buffalo, and various predators of these, such as lions, cheetahs, wild dogs, and hyenas.

Chaparral. Chaparral is a temperate biome that develops in environments where precipitation varies widely from season to season. A common chaparral pattern involves winter rains and summer drought, the socalled Mediterranean climate. Chaparral is characterized by dwarf forests, shrubs, and herbaceous vegetation. This biome is highly prone to wildfire. In North America, chaparral is best developed in parts of the southwest, especially coastal southern California.

Desert. Deserts occur in either temperate or tropical climates. They commonly are found in the centers of continents and in rain shadows of

mountains (a dry region on the side of a mountain sheltered from rain). The most prominent characteristic of a desert is the limited amount of water available. In most cases, less than 25 centimeters (10 inches) of rain fall each year. Not surprisingly, the plant life found in a desert ecosystem is strongly influenced by the availability of water: the driest deserts support almost no plant life, while less-dry deserts may support communities of herbaceous, succulent (having fleshy tissues that conserve water), and annual (returning year after year) plants. In somewhat moister places, a shrub-dominated ecosystem is able to develop.

Semi-evergreen tropical forest. A semi-evergreen tropical forest is a type of tropical forest that develops when a region experiences both wet and dry seasons during the year. Because of this pattern, most trees and shrubs of this biome are seasonally deciduous, meaning that they shed their foliage in anticipation of the drier season. This biome supports a great richness of species of plants and animals, though somewhat less than in tropical rain forests.

Evergreen tropical rain forest. Evergreen tropical rain forests occur in tropical climates with abundant precipitation and no seasonal drought. Because wildfires and other types of catastrophic disturbances are uncommon in this sort of climate regime, tropical rain forests usually develop into old-growth forests. As such, they contain a great richness of species of trees and other plants, a great size range of trees, and an extraordinary diversity of animals and microorganisms. Many ecologists consider the old-growth tropical rain forests the ideal ecosystem on land because of the enormous variety of species that are supported under relatively favorable climatic conditions.

Freshwater biomes

Freshwater biomes can be divided into three general categories: lentic, lotic, and wetlands.

Lentic. A lentic ecosystem is one such as a lake or pond that contains standing water. In lentic systems, water generally flows into and out of the lake or pond on a regular basis. The rates at which inflow and outflow occur vary greatly and can range from days, in the case of small pools, to centuries, in the case of the largest lakes.

The types of organisms that inhabit lentic biomes are strongly influenced by water properties, especially nutrient concentration and water transparency and depth. Waters with a large nutrient supply are highly productive, or eutrophic, while infertile waters are unproductive, or oligotrophic. Commonly, shallow bodies of water are much more productive than deeper bodies of water of the same surface area, primarily because plant growth is influenced by the ability of light to penetrate into the water. Water that becomes cloudy because of the accumulation of silt or dissolved organic matter is likely to have low productivity.

Lotic. A lotic biome is one that consists of running water, as in streams or rivers. The organisms found in a lotic biome depend on factors such as the amount of water in the system, the rate at which it flows, and seasonal changes in the flow rate. Consider a stream in which flooding is common in the spring. Rapidly moving water churns up clay, silt, sand, and other materials from the streambed. The water then becomes cloudy and murky, and light is thus prevented from penetrating it. In this case the stream will not be able to support many kinds of life-forms.

In general, the common lotic ecosystems such as rivers, streams, and brooks are not usually self-supporting in terms of the organisms that live within them. Instead, they typically rely on organic matter carried into them from the land around them or from upstream lakes to support fish and other organisms that live in the biome.

Wetlands. Wetlands are areas that are wet or covered with water for at least part of the year. Some examples of wetlands are marshes, swamps, bogs, sloughs, and fens. Marshes are the most productive wetlands, and are typically dominated by relatively tall plants such as reeds, cat-tails, and bulrushes and by floating-leaved plants such as water lilies and lotus. Swamps are forested wetlands that are seasonally or permanently flooded. In North America, swamps are dominated by tree-sized plants such as bald cypress or silver maple.

Bogs are wetlands that develop in relatively cool but wet climates. They tend to be acidic and, therefore, biologically unproductive. Bogs depend on nutrients obtained from the atmosphere, and are typically dominated by species of sphagnum moss. Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs. Consequently, they are less acidic and more productive than bogs.

Marine biomes

Open ocean. The character of the open-water, or pelagic, oceanic biome is determined by factors such as waves, tides, currents, salinity (salt content), temperature, amount of light, and nutrient concentration. The number of organisms supported by these factors is small and can be compared to some of the least productive terrestrial biomes, such as deserts. The lowest level of food webs in the ocean are occupied by tiny organisms known as phytoplankton. Various species of phytoplankton range in size from extremely small bacteria to larger algae that consist of a single cell and may or may not live in large colonies.

The phytoplankton are grazed upon by small crustaceans known as zooplankton. Zooplankton, in turn, are eaten by small fish. At the top of the pelagic food web are very large predators such as bluefin tuna, sharks, squid, and whales.

The deepest levels of the ocean make up the benthic biome. Organisms in this biome are supported by a meager rain of dead organisms from its surface waters. The benthic ecosystems are not well known, but they appear to be extremely stable, rich in species, and low in nutrient productivity.

Continental shelf waters. Continental shelf waters are areas of ocean water that lie relatively near a coastline. Compared with the open ocean, waters over continental shelves are relatively warm and are well supplied with nutrients from rivers flowing into them. A secondary source of nutrients is water brought to the surface from deeper, more fertile waters that were stirred up by turbulence caused by storms.

Because of the nutrients found in the continental shelf waters, phytoplankton here are relatively abundant and support the larger animals present in the open ocean. Some of the world's most important commercial fisheries are on the continental shelves, including the North and Barents Seas of western Europe, the Grand Banks and other shallow waters of northeastern North America, the Gulf of Mexico, and inshore waters of much of western North America.

Upwelling regions. In certain regions of the ocean, conditions make possible upwellings to the surface of relatively deep, nutrient rich waters. Because of the increased nutrient supply, upwelling areas are relatively fertile, and they support sizeable populations of animals, including large species of fishes and sharks, marine mammals, and seabirds. Some of Earth's most productive fisheries occur in upwelling areas, such as those off the west coast of Peru and other parts of South America and large regions of the Antarctic Ocean.

Estuaries. An estuary is a region along a coastline where a river empties into the ocean. Estuaries display characteristics of both marine and freshwater biomes. They typically have substantial inflows of freshwater from the nearby land, along with large fluctuations of saltwater resulting from tidal cycles. Examples of estuaries include coastal bays, sounds, river mouths, salt marshes, and tropical mangrove forests.

Because the nutrients carried into them by rivers, estuaries are highly productive ecosystems. They provide important habitats for juvenile stages of many species of fish, shellfish, and crustaceans that are later harvested for food. Indeed, estuaries are sometimes called "nursery" habitats.

Seashores. The seashore biome is formed where the land meets the ocean. The specific character of any given seashore biome is determined by factors such as the intensity of wave action, the frequency of major disturbances, and bottom type. In temperate waters, biomes are often characterized by large species of algae, broadly known as seaweed or kelp. In some cases, so-called kelp "forests" can develop, abundant with marine life. In ecosystems characterized by softer bottoms of sand or mud, invertebrates such as mollusks, echinoderms, crustaceans, and marine worms dominate.

Coral reefs. Coral reefs are marine biomes that are unique to tropical seas. They grow in shallow but relatively infertile areas close to land. Corals are small, tropical marine animals that attach themselves to the seabed and form extensive reefs. The physical structure of the reef is provided by the calcium carbonate skeletons of dead corals. Corals live in symbiosis (in union) with algae, and together create a highly efficient system of obtaining and recycling nutrients. For this reason, coral reefs are

highly productive, even though they occur in nutrient-poor waters, and support a great variety of species, including living corals, algae, invertebrates, and fishes.

Human-dominated biomes

Urban-industrial techno-ecosystems. The urban-industrial techno-ecosystem consists of a large metropolitan district that is dominated by humans, human dwellings, businesses, factories, other types of buildings, and roads. This biome supports many species in addition to humans. With few exceptions, however, these species are nonnative plants and animals that have been introduced from other places. These organisms typically cannot live independently outside of this biome, unless they are returned to their native biome.

Rural techno-ecosystems. This anthropogenic biome occurs out-side of intensively built-up areas. This biome is made up of transportation corridors (such as highways, railways, electric power line transmission corridors, and aqueducts), small towns, and industries involved in the extraction, processing, and manufacturing of products from natural resources (such as mining). Typically, this biome supports mixtures of introduced species and those native species that are tolerant of the disturbances and other stress associated with human activities.

Agroecosystems. Agroecosystems are biomes consisting of regions that are managed and harvested for human use. Farms and ranches are examples of agroecosystems. Many agroecosystems are monocultural, consisting of single types of crops, such as corn or wheat, and are not favorable to native wildlife. The objective in such systems is to manage the species in such a way as to produce a maximum dollar profit. Competing species (weeds and insect pests) are destroyed or prevented from growing or surviving. Less-intensively managed agroecosystems may contain mixtures of species, a form of land management known as polyculture. Polyculture systems may provide habitat for some native wildlife species.

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Annelid

Biology
COPYRIGHT 2002 The Gale Group Inc.

Annelid

Annelids, or true-segmented worms, are members of the animal phylum Annelida, the most complex of all wormlike groups of organisms. Annelids are commonly found in terrestrial, as well as marine, brackish, estuarine, and freshwater ecosystems worldwide. Most annelids are free-living, although several species have parasitic, mutualistic, or commensal relationships with other animals, and many species are commonly associated with aquatic and terrestrial plants.

Six major classes comprise this phylum: Polychaeta (polychaete, or many-bristled worms; primarily marine; more than 15,000 species [spp.]); Oligochaeta (oligochaete worms; freshwater, terrestrial, marine; more than 8,000 spp.), Hirudinea (leeches; freshwater, terrestrial, marine; more than 700 spp.), Branchiobdellida (crayfish worms; freshwater, live on crayfishes; more than 100 spp.), Aphanoneura (suction-feeding worms; freshwater; more than 30 spp.), and Acanthobdellida (bristle leech; parasitic on Arctic marine fishes; 1 sp.). As with any group of organisms, the phylogenetic relationships of the diverse groups within annelids, and of the phylum to others within the animal kingdom, is the subject of continuing debate. The marine invertebrate groups Echiura and Sipunculida recently were aligned with the annelids.

All annelids are bilaterally symmetrical , with an elongated, cylindrical body shape divided both externally and internally by a regular, linear series of segments. The highly developed digestive, circulatory, nervous, and excretory systems within the body cavity, or coelom, reflect external segmentation and generally are repeated serially; this is called metameric segmentation, and distinguishes annelids from all other wormlike groups. Annelids range in size from less than 0.7 millimeters (0.019 inch) to over 3
meters (9.8 feet) in length. The number of segments is relatively fixed in some groups (Branchiobdellida, Hirudinea), but indeterminate in others. External form of annelids is diverse, even within each group; the polychaetes may have distinct body regions, with limblike parapodia, chaetae (hairs), tentacles, and antennae, while others may appear similar to an earthworm, with few if any external appendages . Most oligochaete species have chaetae arranged in bundles on each segment. Several aquatic oligochaetes and many polychaetes have gills.

Leeches are usually flattened, with a posterior sucker and anterior suckerlike mouth; several species have jaws, others have an extendable proboscis. The branchiobdellidans have a posterior sucker and an anterior end with several fused segments and distinct teeth. Chaetae are absent in leeches and branchiobdellidans. The single species of Acanthobdellida is shaped like an elongate leech, with a few hooked chaetae located ventrally on a few anterior segments. Annelids are hermaphroditic; reproduction is commonly sexual, but many species reproduce asexually by budding or fragmentation. Annelids are important components of their respective habitats, whether it be the bottom of freshwater or marine environments, or the soil. The feeding habits of many species are important in the decomposition of organic matter and recycling of nutrients in terrestrial and aquatic environments. Many annelids feed on algae, insects, carrion, and other worms, and several leech species consume the blood of turtles, birds, fishes, and mammals.

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Biome

Biology
COPYRIGHT 2002 The Gale Group Inc.

Biome

Earth's major terrestrial, marine, and freshwater ecosystems are known as biomes. They are classified according to similarities in species composition of plants and animals, and by environmental attributes. These attributes include temperature, precipitation, and soil type in terrestrial biomes and temperature, depth, and salinity in aquatic biomes. There are no hard boundaries between biomes and there is much intermixing of species between them.

Biomes are divided into many kinds of ecosystems and habitats, according to local variations in species composition and physical environment (a cloud forest, mud flat, and meadow, to name a few). However, scientists generally recognize between twelve and fifteen major natural terrestrial biomes, including tropical rain forest, tropical deciduous forest, thorn woodland, tropical savanna , desert, sclerophyllous woodland, subtropical evergreen forest, temperate deciduous forest, temperate rain forest, temperate grassland, boreal forest, and tundra. Some scientists consider cultivated land to be a biome. There are seven major freshwater biomes: ice, spring, river, swamp, marsh, lake, and stream. There are six major marine biomes: coral reef; algal bed; estuary; upwelling zone; continental shelf ; and open ocean.

Significant changes in the global environment and climate are causing major shifts in some biomes, such as glacier movement and polar cap melting, and are threatening the survival of others, such as the deforestation of tropical and temperate rain forests.

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biome

biome A biological subdivision of the Earth's surface that reflects the ecological and physiognomic character of the vegetation. Biomes are the largest geographical bioticcommunities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interactions with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical rain forest biome, desert biome, tundra biome.

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Biome

Plant Sciences
COPYRIGHT 2001 The Gale Group Inc.

Biome

A biome is a particularly useful biogeographic unit that results from large-scale climatic patterns. A portion of a biome, such as the tropical rain forest found in the Amazon basin, may cover thousands of hectares. It will have
recognizable vegetation features that impart a sameness that defines the unit. The fact that this region differs from others, such as the desert biome, is evident to even the most untrained observer. Frederic Clements (1874-1945) and Victor Shelford (1877-1968) observed in 1939 that not only was the vegetation characteristic, but that these were plant-animal formations with associated fauna. In the 1960s an ecological research effort, the International Biological Program, sought to characterize the structure and function of major biomes through intense study. American botanist and ecologist Robert Whittaker (1920-1980) summarized some characteristics of the biomes in his book Communities and Ecosystems, in which he arranged them graphically according to interacting gradients of mean annual temperature and moisture availability (see accompanying figure).

The Influence of Rainfall

You can bring this model to life though observation as you travel within the continental United States. The Interstate Highway System allows easy movement across the country along latitudes, parallel to the equator, or the longitudes, perpendicular at the equator. Travel on Interstate 80 from New York City to San Francisco during the summer growing season reveals an obvious gradient of change in vegetation. The deciduous forests of central New York State are lush green in most years, with trees predominating where human development has not occurred. In the Midwest, traveling from Illinois into Iowa and Nebraska, the trees are absent except along rivers, and grasses and broad-leaved nonwoody forbs cover the landscape like a carpet.
Much of this region is now dominated by fields of grain such as corn and wheat, domesticated replacements of the native vegetation that once was predominant in the grasslands. (Remnants of the native prairie are rare.) In August, the green prairies begin to show some signs of the golden color of mature fields as the various annual grasses produce seeds that will ultimately produce plants for the following year. Meanwhile the perennials will die back to ground level by late fall, and then start growth again the following year.

The farther west one travels, the sparser the vegetation and the less luxuriant the crops, until dry short-grass prairies merge into deserts and badlands with more widely spaced plants and soil that is frequently bare. The predominant color throughout the year in the deserts tends toward earth tones of beige and brown with small green leaves being less evident. Two major north-south mountain barriers, the Rocky Mountains and the Sierra Nevada range in California, interrupt the gradual transition. In these areas, elevation-dependent differences produce a complex mosaic of vegetation that does not fit the overall pattern. Finally, on the West Coast, moisture-laden air deposits rain on the windward sides of the coastal mountains and abruptly changes the pattern. Lush temperate rain forest vegetation forms a margin along the western edges of California north of San Francisco, just as in coastal Oregon and Washington. Southward, the forests are replaced by coastal shrub lands because rainfall is not sufficient to support extensive forests in most places, and the inland deserts are even closer to the ocean shores. This east-west gradient across the country parallels a decline in moisture availability, the first of two major climatic factors that determine the character of biomes.

The Influence of Temperature

A similar trip along Interstate 75 in January from the Canadian border at Sault Ste. Marie, Michigan, to Florida provides evidence of the second major climatic factor gradient. In northern Michigan, it is likely that the ground will be snow covered most of the month. Ohio and Kentucky can present occasional periods with persistent snow cover; by the time you reach Georgia, however, snow generally disappears shortly after it falls, and Florida has only rain. This gradient is not so much a result of differences in volume of precipitation (the amount of rain or snowfall), but the prevailing temperatures. When the north to south temperature gradient overlays the east to west moisture gradient, the type of biological community that will be supported is determined. Similar interactions of temperature and moisture availability determine the presence of other biomes globally as indicated in the map of their distribution. Toward the poles, Canadian boreal forests change over to tundra and illustrate the results of extending the temperature gradient.

Tropical rain forests occur where the temperature is greater near the equator and daily tropical rains maintain high moisture levels. This tropical region along the equator forms a boundary with the temperature gradients in the Southern and Northern Hemispheres being mirror images of each other. In the tropical forest biomes, there is little seasonal change; the prevailing conditions are wet and warm. Seasonal temperature variation is minimal in regions adjacent to tropical forests, but ultimately at increasing distance from the equator, alternating wet and dry periods produce fluctuations in growth rates in subtropical seasonal forests. This moisture-dependent alternation
is different from the temperate biomes covering the United States and Canada where the seasons are produced by the alternation of a cold winter and relatively warmer summer.

The classification of biogeographic areas into biomes is only one of several different methods of organizing terrestrial environments. Michael Barbour, in Terrestrial Ecosystems, gives a brief historical review of other classification systems, and Robert Bailey, in Ecosystem Geography, updates various ways of classifying the Earth's surface. Barbour uses the term vegetation types to classify landscape patterns and points out significant differences between the various subdivisions that can be identified, while Bailey prefers the term ecoregion as the unit of subdivision at the level of assemblages of landscapes.

Since the distribution of biomes is dependent on global climate patterns, one may question what impact global climatic change could have on these biogeographic units. A continuation of global warming would change the boundaries by moving warmer conditions closer to the poles.

In addition, the climatic changes do not just involve changes in temperature; they also involve changes in air movement patterns, alteration of temperatures (known as El Niño and La Niña) within the oceans, and storm distributions that change the annual growth of both plants and animals. This is not a new phenomenon on Earth. Cold periods have existed in the past, and biome boundaries have shifted in response. This is known from the pollen record left in lakes and bogs over many thousands of years. The challenge to plants and animals is not so much the extent or duration of expected temperature changes, but the rate of those changes. Vegetation can respond to changes that take hundreds or thousands of years to occur by gradually dispersing into new areas or evolving. It cannot move or reproduce fast enough to adapt to the same changes if they occur over tens or hundreds of years. The total ecologic impact of these potential rapid changes is not currently known.

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biome

biome A biological subdivision that reflects the ecological and physiognomic character of the vegetation. Biomes are the largest geographical biotic communities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interaction with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical rain-forest biome, desert biome, tundra biome.

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biome

biome A biological subdivision of the Earth's surface that reflects the character of the vegetation. Biomes are the largest geographical biotic communities that it is convenient to recognize. They broadly correspond with climatic regions, although other environmental controls are sometimes important. They are equivalent to the concept of major plant formations in plant ecology, but are defined in terms of all living organisms and of their interactions with the environment (and not only with the dominant vegetation type). Typically, distinctive biomes are recognized for all the major climatic regions of the world, emphasizing the adaptation of living organisms to their environment, e.g. tropical-rainforest biome, desert biome, tundra biome.

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freshwater fish

freshwater fish Fish that live only in fresh water. Many families and orders of fish are considered to include only primary freshwater species (i.e. species that evolved without contact with a marine environment). Some families have a worldwide distribution (e.g. the Cyprinidae are found in freshwater basins all over the world, except for South America and the Australian region). Others (e.g. the Cobitidae, loaches) are found only in the Eurasian region. A few families have a very restricted distribution (e.g. the Amiidae (bowfin) are found only in the eastern half of the USA).

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freshwater fish

freshwater fish Fish that live only in fresh water. Many families and orders of fish are considered to include only primary freshwater species, i.e. species that evolved without contact with a marine environment. Some families have a worldwide distribution, e.g. the Cyprinidae are found in freshwater basins all over the world, except for S. America and the Australian region. Others, e.g. the Cobitidae (loaches), are found only in the Eurasian region. A few families have a very restricted distribution, e.g. the Amiidae (bowfin) are found only in the eastern half of the USA.

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biome

biome A major ecological community or complex of communities that extends over a large geographical area characterized by a dominant type of vegetation. The organisms of a biome are adapted to the climate conditions associated with the region. There are no distinct boundaries between adjacent biomes, which merge gradually with each other. Examples of biomes are tundra, tropical rainforest, taiga, chaparral, grassland (temperate and tropical), and desert.

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annelid

annelid Any member of the Annelida phylum of segmented worms. All have encircling grooves usually corresponding to internal partitions of the body. A digestive tube, nerves, and blood vessels run through the entire body, but each segment has its own set of internal organs. The three main classes are: Polychaeta (marine worms), Oligochaeta (freshwater or terrestrial worms), and Hirudinea (leeches).

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biome

biome Extensive community of animals and plants whose make-up is determined by soil type and climate. There is generally distinctive, dominant vegetation, and characteristic climate and animal life in each biome. Ecologists divide the Earth (including the seas, lakes, and rivers) into ten biomes.

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.